A black hole is an object in the universe that is so heavy it attracts everything around, including light, and thus appears as a "black hole".
The weight of an object is due to a force that attracts it, and depends on the mass of the object, and the mass of the object that attracts it. On Earth, for example, an object would fall because of its weight, because it is attracted by the center of the Earth. The heavier an object is, the more it will be attracted by the Earth. The light is made of tiny particles, the photons, that are so light (they almost have no mass...), that they almost have no weight. However, they have a very high speed (photons move at the speed of light). They move so fast, and they are so light, that they can't "fall" : their weight is far too small. In black holes, however, the black hole has such a great mass, that everything coming close from it is attracted, and has an immense weight, even the light. For someone looking at a black hole from a distance (better not to be too close, in this case !), he will see no light coming out from it, since the light itself is attracted. Thus the black hole appears... black.